Intervertebral discs comprise a highly organized matrix of collagen, water, and proteoglycans produced by differentiated chondrocytes. Each intervertebral discs comprises a central highly hydrated and gelatinous nucleus pulposus (nucleus) surrounded by an elastic and highly fibrous annulus fibrosus (annulus). Cartilaginous endplates provide a connection to the vertebrae inferiorly and superiorly to the intervertebral disc. This cushioned arrangement within the intervertebral discs allows the discs to facilitate movement and flexibility within the spine while dissipating hydraulic pressure through the spine.
Intervertebral discs are susceptible to a variety of derangements from degenerative disease and traumatic injury that may result in molecular and morphological changes that affect the macromolecular structure of the disc. These derangements may cause an at least partial collapse and a loss of height of the intervertebral disc with consequent compression of spinal nerves and pain. For example, degenerative disc disease may occur as an age-related process in which the nucleus changes from a gelatinous material with high water content to a more fibrous, water-depleted material that may form fissures and/or tears. The degenerated nucleus may exhibit a decreased ability to evenly distribute hydraulic pressure from the compression of the spine through the intervertebral disc and may prolapse into the surrounding annulus. Degenerative disc disease may also result in tearing of the vertebral endplates and/or the annulus tissue, causing the nucleus to herniate through the fibers of the annulus to compress spinal nerve roots and cause pain.
Derangements of the intervertebral disc may cause severe back pain, spasms of back muscles, muscle weakness in the legs, numbness in the leg and/or foot, radiating pain down the leg, and changes in bladder and/or bowel function. Pain from intervertebral disc derangements may be intractable and debilitating. Pain may be improved for some patients with physical therapy, modification of activity, and/or medication. Patients that fail to respond to noninvasive interventions for back pain, however, may require surgery on the damaged intervertebral disc.
A variety of surgical interventions may be employed to relieve nerve pressure and pain. For example, one possible treatment may comprise a discectomy wherein a herniated portion of the intervertebral disc is removed. In another procedure, a laminectomy may be performed to remove a portion of the lamina to enlarge the spinal canal and relieve nerve pressure. In a spinal fusion procedure, two or more vertebrae may be permanently fused in the area of the damaged disc to eliminate compression of the damaged disc caused by motion.
Surgical intervention for intervertebral disc derangements may also comprise replacing the damaged disc with an artificial disc in an arthroplasty surgery. Arthroplasty may be preferred to a spinal fusion procedure in some patients because the artificial disc is intended to restore and preserve the native biomechanics of the intervertebral disc, such as providing the requisite cushion to the adjacent vertebra, supporting unrestricted motion of the spine, and reducing or preventing the degeneration of adjacent intervertebral discs, which may be damaged after fusion surgeries due to the permanently altered motion characteristics of the spine in the fused area.
An artificial disc may comprise a variety of biocompatible materials. For example, one type of artificial disc comprises a sliding polyethylene core sandwiched between cobalt chromium alloy endplates. Adverse complications associated with artificial discs include disc migration causing nerve compression which requires revision surgery, degeneration of discs at another level of the spine, subsidence of the artificial discs, facet joint arthrosis, and wear of the polyethylene in the artificial disc. Measures to correct these problems may require a subsequent surgery for removal of the artificial disc and fusion of the affected vertebrae.